1. Field of the Invention
The present invention relates to a high-speed optical scanning device, wherein a spherical exit window symmetrically surrounds a rotatable scan optic, for use in cylindrical field imaging applications and other wide scanning angle imaging applications in which scanning angles of up to 360.degree. without asymmetric optical aberration are desirable.
2. Description of the Prior Art
In a typical laser imaging application, light beams produced by a laser light source, such as a helium-neon gas laser or a laser diode, are reflected from the reflective surface of an optical scanning device's rotatable scan optic and onto an imaging surface lined with photo-sensitive material, such as photo-sensitive paper or film. As a result, the light beams produce a two-dimensional image on the photo-sensitive material, consisting of a series of small dots.
The imaging surface itself may either be flat or curved, depending on the optical design configuration of the particular optical scanning device being utilized. In a typical cylindrical field imaging application, for example, the photo-sensitive material is first loaded onto the inside surface of a hollow cylindrical drum. Then, the optical scanning device is precisely moved along rails at a constant speed along the center axis of the drum in such a way that the photo-sensitive material attached to the surface of the drum is scanned and exposed to the light beams reflected by the rotating scan optic of the optical scanning device. The photo-sensitive material itself may either be retained on the outer surface of the drum, if the drum is composed of a transparent material, or the inner surface of the drum. In some alternative cylindrical field imaging applications, however, the scan optic is instead non-rotatable. In such alternative applications, the photo-sensitive material retained on the surface of the drum is successfully scanned via the drum's rotating about the non-rotatable scan optic as the optical scanning device moves along the center axis of the drum.
The rotatable scan optic of an optical scanning device typically consists of a single mirror, an assembly of more than one mirror, or a glass prism. The rotatable scan optic is commonly mounted on a shaft that is supported by some type of bearing assembly which includes radial and thrust bearings. The shaft itself is ultimately rotated by a motor which is driven by a controlling electronics system. See, for example, U.S. Pat. No. 4,726,640 to Iwama et al. Iwama et al is directed to the problem of wobble in the rotational axis of the rotatable scan optic as the rotatable scan optic rotates during a scanning operation, the extreme difficulty and high cost involved with producing precisely manufactured stationary and rotary shafts which require complicated grooving work and very accurate surface machining which is often associated with dynamic pressure pneumatic bearing assemblies, the problem of parts being worn down due to frictional contact commonly associated with dynamic pressure bearing assemblies at the beginning and end of each scanning operation, the extreme difficulty and high cost associated with maintaining balance of the rotatable scan optic and its bearing assembly, and obtaining overall device compactness. To overcome these problems, Iwama et al teach an optical scanning device which incorporates a dynamic pressure pneumatic bearing as a radial bearing and a magnetic bearing as a thrust bearing. In particular, the disclosed device teaches a hollow rotary shaft, integrated with a rotatable scan optic, and a stationary shaft disposed within the rotary shaft, the outer periphery of the stationary shaft and the inner periphery of the rotary shaft cooperatively constituting a dynamic pressure pneumatic bearing, an annular rotor magnet assembly mounted on the outer periphery of the rotary shaft adjacent to a lower end of the rotary shaft and magnetized to different polarities sequentially in a circumferential direction, a first rotary magnet mounted in the upper end portion of the rotary shaft, and a first stationary magnet mounted in an upper end portion of the stationary shaft to face the first rotary magnet with the same polarity as that of the first rotary magnet, the first rotary magnet and the first stationary magnet constituting a thrust magnetic bearing due to a repulsive force action therebetween.
U.S. Pat. No. 4,805,972 to Tanaka et al is a device which utilizes dynamic pressure gas bearings in order to confront the problem of wobble. In particular, wobble in such bearing assemblies is caused by error in the machining of the ball bearings, vibration caused by passage of the balls within the ball bearing assembly, and vibration caused by the retainer or the irregularity of rotation caused by the grease enclosed in the ball bearing assembly. As a result of such wobble, the life of the ball bearings is shortened due to the friction they are exposed to when the rotatable scan optic rotates at very high speeds. Another problem involves lubricants, such as grease, which are used within the ball bearing assembly staining the reflective surface of the rotatable scan optic. To overcome these problems, Tanaka et al teach the incorporation of dynamic pressure gas bearings as thrust and radial bearings instead of the ball bearings. In particular, the disclosed device teaches a dynamic gas bearing device in a rotatable unit in which a rotational member, put on a cantilevered fixed shaft, is designed such that an operating gas, generated by a dynamic pressure groove formed between the fixed shaft and the rotational member, is directed into a pressure chamber between the fixed shaft and the rotational member and supports the rotational member in the thrust direction and that the pressure in the pressure chamber is adjusted by a hole formed in the fixed shaft or the rotational member.
U.S. Pat. No. 4,934,836, also to Tanaka et al, is directed to the problems of radially balancing the weight of the rotatable scan optic and its bearing assembly, the extreme difficulty and high cost associated with machining a shaft assembly with precise machining accuracy, reducing the thrust load, and reducing frictional contact in the radial and thrust bearing surfaces of the bearing assembly at the beginning and end of scanning operations. To overcome these problems, Tanaka et al teach a reduced weight rotary member for reducing thrust load and radial/thrust bearings which are of the dynamic pressure type fluid bearing. In particular, the disclosed device teaches a housing which has a vertically extending cylindrical bore, with a radial bearing surface formed on the inner peripheral surface, and a thrust bearing surface formed on the inner bottom surface of the cylindrical bore. A shaft member, which is supported rotatably in the cylindrical bore of the housing, has a radial receiving surface and a thrust receiving surface respectively formed on the outer peripheral surface and the bottom surface. A dynamic pressure generating groove of a spiral shape is formed in at least one of the radial bearing surface and the radial receiving surface. When the shaft member is rotated, a gas in the housing is sucked by the pumping action of the dynamic pressure generating groove and flows into a pressure chamber between the thrust bearing surface and the thrust receiving surface through a radial space between the radial bearing surface and the radial receiving surface. In this way, the pressure in the pressure chamber can maintain the shaft member in a floating position at a predetermined vertical height.
U.S. Pat. No. 5,046,797 to Kirusi utilizes a radial air bearing and a magnetic protective cover in order to overcome problems associated with oil mist contaminating the reflective surface of the rotatable scan optic.
Finally, U.S. Pat. No. 5,069,515 to Itami et al is directed to the problems of correcting the weak rigidity of the thrust bearing, making the bearing assembly so that the optical scanning device is compact, reducing the lengthy time and difficulty in preparing/processing a rotary shaft, and reducing overall device cost and unnecessary parts. To overcome these problems, Itami et al teach achieving device compactness and reducing parts by reducing the axial length of the optical scanning device and overcoming a weak thrust bearing and difficulty in preparing a rotary shaft by teaching a hollow rotary shaft, with radial air bearings and a three-magnet thrust bearing or dynamic pressure thrust air, on top of a fixed shaft. In particular, the reference teaches an optical deflector of an air bearing type which has a fixed shaft; a rotary shaft having a hollow portion fitted onto the fixed shaft; a support device disposed between an end portion of the fixed shaft and an end portion of the hollow portion opposite thereto and supporting the rotary shaft in an axial direction thereof; a radial air bearing formed between an inner circumferential face of the hollow portion and an outer circumferential face of the fixed shaft; a polygon mirror fixed to the rotary shaft; a driving device for rotating the rotary shaft; and a device for deflecting light irradiated onto the polygon mirror by rotating the rotary shaft through the driving device. The support device has a first magnet directly attached onto an end face of the fixed shaft, a second magnet opposite to the first magnet and attached to the hollow portion of the rotary shaft, and a third magnet opposite to the second magnet and attached to a casing for covering the polygon mirror. Magnetic poles of the first, second, and third magnets are opposite to each other to generate magnetic repulsive force.
In order to obtain a high resolution image on a given imaging surface in as short amount of time as possible, it becomes necessary to rotate the rotatable scan optic of an optical scanning device at a very high rotational speed, typically on the order of twenty thousand (20,000 rpm) revolutions per minute or higher. The rotating speed of an optical scanning device's scan optic is commonly referred to as that device's "scan rate." Thus, the image-producing productivity of a given optical scanning device is largely dependent upon the scan rate of the device's scan optic, for a higher scan rate generates high-resolution images faster. However, higher scan rates also generate numerous undesirable side effects which are magnified as rotational speed increases above 20,000 rpm.
The worst of these undesirable side effects include the problems of loud noise, excessive wobble, scan optic contamination, and scan optic abrasion. The problem of loud noise arises when excessive wind turbulence is generated by a scan optic rotating at a high rotational speed. Such wind turbulence often produces a high-pitched noise which may be extremely irritating to an operator of the optical scanning device. The problem of excess wobble arises when excessive wind turbulence generated by a scan optic rotating at a high rotational speed causes a random jitter or wobble in the scan optic's rotational axis. Such wobble often produces undesirable perturbations in the scan lines which are formed on the imaging surface, thus degrading an image's overall quality. The problem of scan optic contamination arises when excessive wind turbulence generated by a scan optic rotating at a high rotational speed causes dust and other contaminants to swirl and accumulate on the reflective surface of the rotatable scan optic. Such contamination can degrade overall image quality. The problem of scan optic abrasion occurs over time such that the reflective surface is abraded by the swirling dust and other contaminants which strike the reflective surface of the scan optic at high speed. Eventually, the cumulative effects of these collisions deteriorate the reflective surface, and can likewise degrade overall image quality. In short, these problems tend to escalate as scan rates for optical scanning devices are increased.
In early attempts to solve some of these problems, high-speed optical scanning devices consisting of streamlined rotatable scan optics were developed as a solution for reducing wind turbulence in order to ultimately reduce loud noise, excessive wobble, scan optic contamination, and scan optic abrasion problems. In addition, aerodynamic shields and baffles which serve to control air flow about the rotatable scan optics were developed as well. Even sound-deadening foam was used in an attempt to further reduce the loud noise problem. However, when such solutions are incorporated in a given optical scanning device, they each add significant cost to the production of the device. This fact along with the continuing modern trend toward optical scanning devices having even faster scan rates renders such solutions as not only economically unfeasible but also technically unfeasible, for any modern high-speed optical scanning device which incorporates such solutions will be difficult to produce and most likely will be priced out of the modern market.
In more recent attempts to solve the known problems associated with high-speed optical scanning devices, it has been demonstrated that wind turbulence generated by rotating the rotatable scan optic of an optical scanning device can be substantially reduced by housing the rotating scan optic in an enclosure designed in such a way that the enclosure's internal surfaces are in close proximity to the rotating surface(s) of the rotating scan optic. See, for example, U.S. Pat. No. 4,610,500 to Kramer which is directed to the problem of optical scanning devices being uniquely and specifically designed for very narrow applications and not being adaptable, as a unitary device, for various applications which depend on the specific wavelength of laser light which is used, imaging surface characteristics, scan length, scan resolution, and space which is available to accommodate the optical scanning device itself. To overcome these problems, Kramer teaches an optical scanning device, as a unitary device when fully assembled, having a rotatable scan optic, a drive motor, and light beam focusing optics which will meet the specific requirements of many different applications, such requirements including specific laser wavelengths, resolutions, and scan angles. In particular, the disclosed device teaches a unitary assembly which includes a rotatable scan optic, a motor for rotating the rotatable scan optic, and a lens for focusing a deflected laser beam on an image surface. A housing, including a base plate and a cover removably mounted on the base plate, encloses the rotatable scan optic. The base plate has a surface with respect to which the position of the rotatable scan optic and lens is referenced. The cover has a platform on which lenses are interchangeably mounted such that each lens has the correct position and orientation with respect to the entrance pupil of the optical scanning device. The rotatable scan optic is also removably mounted on the shaft of the motor and referenced against a shoulder of the shaft which is precisely spaced with respect to the reference surface of the base plate of the housing. The unitary assembly may be installed in a laser printer, or other device requiring a scanner, aligned with the light beam of the laser light source which enters an opening in the housing below the rotatable scan optic, a hologon disc. The light beam is deflected and scanned by the facets of the rotatable scan optic and focused by the lens onto an imaging surface, such as the drum or belt on which a latent electrostatic image is formed by the scanned light beam.
U.S. Pat. No. 5,084,883 to Khalid et al, is directed to the problems of realignment difficulty when a component of an optical scanning device needs to be replaced, poor light beam quality commonly produced by laser diode light sources, and preventing bearing assembly lubricant particles from getting onto the reflective surface of the rotatable scan optic and scan lens. To overcome these problems, Khalid et al teach an optical scanning device which does not require realignment when a device component needs to be replaced, manipulating and modifying the light beam emitted from a laser diode in an inexpensive manner so that it can be used in high quality imaging, and mounting the laser light source, rotatable scan optic, and scan lens to a support body in a gas tight manner. The support body is preferably provided with a conduit for gas flow communication, specifically for introducing a gas under pressure and maintaining a positive pressure inside the support body. This ensures that any lubricant particles from the bearing assembly will not enter the support body and contaminate the reflective surface of the rotatable scan optic and/or the scan lens.
By enclosing the rotatable scan optic of an optical scanning device in an enclosure in such a way that the enclosure's internal surfaces are in close proximity to the rotating surfaces of the rotating scan optic, the wind turbulence generated by the rotating scan optic can be substantially reduced. Particularly, the ambient air within such an enclosure tends to flow uniformly in a circular fashion along with the surface of the rotating scan optic as the scan optic rotates about its rotational axis, thus substantially reducing wind turbulence directed toward the scan optic's rotational axis. The direct results of such a reduction in wind turbulence are less noise, reduced wobble, negligible scan optic contamination, and reduced scan optic abrasion.
In an effort to include a scan optic enclosure in an optical scanning device, many modern optical scanning devices have been designed with scan optic enclosures incorporating entrance and exit windows that allow light beams produced by laser light sources to enter the enclosure, be reflected from the rotating scan optic, and exit the enclosure for imaging. Such modern devices typically are designed such that the entrance and exit windows are flat. Utilizing flat windows, however, gives rise to a significant problem if the optical scanning device is to be utilized in cylindrical field imaging applications and other wide scanning angle imaging applications, particularly if the exit window is flat. For example, utilizing a flat exit window in an optical scanning device generally limits the device to applications merely requiring small scanning angles of less than 60.degree.. Thus, whereas a design incorporating a flat exit window may be, for the most part, acceptable for optical scanning devices utilized in flat field imaging applications which merely necessitate small scanning angle capability, such a design is not acceptable for cylindrical field imaging applications and other wide scanning angle applications requiring scanning angles greater than 60.degree. and up to 360.degree..
A potential design solution to the problem of including a scan optic enclosure without limiting wide scanning angle capability would be to fully surround the rotating scan optic with a cylindrical exit window. Although such a design solution would provide an optical scanning device with large scanning angle capability, the cylindrical shape of the exit window would introduce undesirable asymmetric aberrations to the laser light beams as they passed through the cylindrical exit window, thus ultimately degrading image quality. Such asymmetric aberrations are indeed a significant problem, for they cannot be easily corrected in a conventional optical system.
In sum, the prior art has yet to develop an optical scanning device which effectively prevents wind turbulence, loud noise, wobble, scan optic contamination, and scan optic abrasion and which simultaneously possesses wide scanning angle capability of up to 360.degree. without asymmetric aberrations. It is the object of the present invention to address and solve this problem.